S/390: Improve error checking for optional operands

S/390: Improve error checking for optional operands

So far we only had an instruction flag which made an arbitrary number
of operands optional.  This limits error checking capabilities for
instructions marked that way.  With this patch the optparm flag only
allows a single optional parameter and another one is added (optparm2)
allowing 2 optional arguments.  Hopefully we won't need more than that
in the future. So far there will be only a single use of optparm2.

gas/ChangeLog:

2017-05-30  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* config/tc-s390.c (md_gather_operands): Support new optparm2
	instruction flag.

include/ChangeLog:

2017-05-30  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* opcode/s390.h: Add new instruction flags optparm2.

opcodes/ChangeLog:

2017-05-30  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* s390-dis.c (s390_print_insn_with_opcode): Support new optparm2
	instruction flag.
	* s390-mkopc.c (main): Recognize the new instruction flag when
	parsing instruction list.